Iron-nickel alloys intergranular corrosion

This alloy has a nominal composition of 65% nickel, 28% molybdenum and 6% iron. It is generally used in reducing conditions. It is intended to work in very severely corrosive situations after post-weld heat treatment to prevent intergranular corrosion. These alloys have outstanding resistance to all concentrations of hydrochloric acid up to boiling-point temperatures and in boiling sulfuric acid solutions up to 60% concentration. 

The composition of this alloy (54% nickel, 15% molybdenum, 15% chromium, 5% tungsten and 5% iron) is less susceptible to intergranular corrosion at welds. The presence of chromium in this alloy gives it better resistance to oxidizing conditions than the nickel/molybdenum alloy, particularly for durability in wet chlorine and concentrated hypochlorite solutions, and has many applications in chlorination processes. In cases in which hydrochloric and sulfuric acid solutions contain oxidizing agents such as ferric and cupric ions, it is better to use the nickel/molybdenum/ chromium alloy than the nickel/molybdenum alloy. 

Henthorne, M., Intergranular Corrosion of Iron and Nickel Base Alloys, Localised Corrosion-Cause of Metal Failure, ASTM STP 516, ASTM, 66 (1972)... 

Table 19.3 Summary of chemical tests used for the determination of susceptibility to intergranular corrosion of iron-nickel Chromium alloys ...

Cowan, R. L. and Tedmon, C. S. (Jr.), Intergranular Corrosion of Iron-Nickel-Chromium Alloys , in Advances in Corrosion Science and Technology, Vol. 3, (eds. M. G. Fontana and R. W. Staehle), Plenum Press (1973)... 

Intergranular corrosion can also occur in other metals and alloys in connection with grain boundary precipitation, for example, in aluminum magnesium (AlMg) 5 alloy, nickel chromium 30 iron (NiCr30Fe), or Ti, and in methanol with traces of water and HCl. Solution annealed austenitic chromium-nickel steels can suffer intergranular corrosion from strong oxidants such as chromium... 

Figure 9-12. Left field fracture of a gold-plated iron-nickel-cobalt alloy. Fracture occurred at the metal/glass interface (arrow points to fracture). Right stress-corrosion cracking in an iron-nickel-cobalt alloy. Both transgranular and intergranular cracks are observed (arrows).

Nickel-iron-chromium (for example, aUoy 800) and nickel-chromium-iron (alloy 600) may become sensitized by precipitation of chromium carbides and subject to intergranular corrosion... 

Intergranular and transgranular cracking often occur simultaneously in the same alloy. Such transitions in crack modes are observed in alloys with high amounts of nickel, iron chromium and brasses. In corrosion under tension, ruptures are fragile and are sometimes characterized by the presence of cleavages, notably in the case of hydrogen embrittlement.16... 

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